Scintillation detectors are used in medicine to detect radiation emitted from a patient as a result of an internally administered radiopharmaceutical or emitted from a source external to the patient. Such detectors are used in many modern medical imaging techniques including computed tomography (CT), single photon emission computed tomography (SPECT), and positron emission tomography (PET). Scintillation detectors include a scintillator, usually a scintillation crystal, and one or more photomultiplier tubes (PMTs) or other photo sensors to locate the origin and determine the energy of a gamma ray or other incident radiation. In the simplest case, when a gamma ray interacts with a scintillation crystal, the gamma ray ejects an energetic electron. If the gamma ray is completely absorbed by a photoelectric interaction, the ejected electron is called a photoelectron. As the ejected electron returns to its rest energy level, one or more photons are emitted. For typical scintillation crystals, the emitted photons are in the visible spectrum (light). Medical imaging systems create an image by recording the location of each flash of visible light in the scintillation crystal and then calculating the location and shape of the source of gamma rays that generated the flashes which may be a tumor or other body part of a patient who has been treated with a radiopharmaceutical.
The resolution of a scintillation detector can be improved by the use of wavelength-shifting optical fibers to capture photons emitted by the scintillation crystal. These fibers can be laid in orthogonal layers of fibers. A PMT connected to the end of each fiber provides a signal when a visible-light photon is captured and propagated through the fiber to the PMT. When the PMTs connected to an orthogonal pair of fibers record photons at the same time, the source of the gamma ray is determined to be at the intersection of the two fibers. One such system is shown in U.S. Pat. No. 5,600,144. While using wavelength-shifting optical fibers can improve the image resolution of a detector, room for improvement in the intrinsic spatial resolution of the detector (.DELTA.x) and the intrinsic energy resolution of the detector (.DELTA.E/E) remains.
In this specification and the accompanying claims the term "radiation" is meant to include any form of high-energy rays. For instance, electromagnetic radiation such as gamma radiation (high energy electromagnetic photons), alpha radiation (helium nuclei), beta radiation (high energy electron radiation) and x-rays. Gamma rays are used throughout as exemplary because they are widely used in medical imaging.
When a gamma ray interacts with a scintillation crystal, the crystal gives off light (photons in the visible spectrum) equally in all directions (isotropically). These photons primarily have a wavelength, .lambda..sub.0, which is a characteristic of the crystal material, and particularly, is influenced by dopant chemicals added to the crystal to control the crystal's scintillation properties.
Photons emitted by the scintillation crystal at a wavelength, .lambda..sub.0, may strike one of the wavelength-shifting fibers which are adjacent the crystal. If the incoming photons strike the fiber at an angle of incidence which is greater than some critical angle, the photons will be reflected and so will not enter the fiber. On the other hand photons traveling on a path that is at an angle less than the critical angle will enter the fiber. Once inside a wavelength-shifting optical fiber, some of the photons may be absorbed and re-emitted primarily at a longer wavelength, .lambda..sub.1. (This shift from an incoming wavelength, .lambda..sub.0, to a longer, re-emitted wavelength, .lambda..sub.1, is the source of the name "wavelength-shifting fiber".)
Re-emission within the fiber at .lambda..sub.1 is also isotropic, and so it results in a change of direction of the photon. Most of the re-emitted photons escape, passing through the wall of the optical fiber. Only those photons that happen to be re-emitted at a sufficiently acute angle with respect to the axis of the fiber will undergo total internal reflection so as to be transmitted the length of the fiber to the PMT or other photosensor at the fiber's end. The rest of the re-emitted photons pass through the walls of the fiber and never reach the photosensor at the end of the fiber.
Because of various well known physical factors, the overall efficiency (.epsilon.) of delivering photons to the ends of the fiber is limited for this system, typically approaching 8% (.epsilon..ltoreq.0.08). The remaining 92% of the re-emitted photons escape and go undetected. This inefficiency is a major factor limiting spatial and energy resolution of a detector using wavelength-shifting optical fibers. The statistical limit of spatial resolution (.DELTA.x) of the detector is inversely proportional to the square root of the number of photons available to be trapped multiplied by the trapping efficiency. Mathematically this may be expressed as: EQU .DELTA.x.varies.1/(.epsilon.N).sup.1/2
where N is the number of photons emitted by the crystal and .epsilon. is the capturing efficiency. Similarly the energy resolution (.DELTA.E/E) of a detector is inversely proportional to the square root of the number of photons available to be trapped multiplied by the trapping efficiency. Mathematically this may be expressed as: EQU .DELTA.E/E.varies.1/(.epsilon.N).sup.1/2
Another factor which affects the efficiency of the absorption process is the bandwidth of the light surrounding .lambda..sub.0 and .lambda..sub.1. Neither of these two wavelengths is a single value. Instead, the photons emitted by the scintillating crystal and the photons re-emitted by the wavelength-shifting optical fiber have a range of wavelengths centered about these values. Indeed, this is true of all the wavelengths discussed in this specification.
As noted above, .lambda..sub.0 is a characteristic of the scintillating crystal related to the atomic structure of the chemicals composing the crystal. Similarly, the wavelength at which the wavelength-shifting optical fiber absorbs, .lambda..sub.0, and its re-emission wavelength, .lambda..sub.1, are characteristics of the wavelength-shifting optical fiber. Specifically, the change in wavelength (.lambda..sub.1 -.lambda..sub.0) in a wavelength-shifting optical fiber is influenced by a dopant in the fiber. Different dopants cause the fiber to absorb photons of different wavelengths and re-emit at other characteristic wavelengths. Suitable dopants include but are not limited to bis-MSB which has an absorption peak at 345 nm and an emission peak at 420 nm (blue) and the fluor K-27 which has an absorption peak at 427 nm and an emission peak at 496 nm (green). Suitable wavelength shifting fibers can be purchased from Bicron Corporation in Newbury Ohio and Kuraray Corp. of Japan.
Because of the bandwidths associated with each of the various wavelengths, it is important to select a wavelength-shifting optical fiber with characteristics that allow it to absorb the visible photons from the scintillating crystal and re-emit photons at a longer wavelength, and it is important that the bandwidths of the absorption and re-emission do not overlap substantially. The reason for this is clear. If the wavelength-shifting optical fiber happens to emit a photon at the high (short) end of its re-emission bandwidth, .lambda..sub.0 -.DELTA..lambda.=.lambda..sub.3, and .lambda..sub.3 happens to be within the bandwidth that the wavelength-shifting optical fiber will absorb, that photon can be reabsorbed by the optical fiber. If this occurs, that particular photon has only a very small chance of being conducted by the fiber to its end for detection. Accordingly, where there is substantial overlap between the absorption and re-emission wavelengths of the wavelength-shifting optical fiber, the efficiency, .epsilon., decreases and the quality of the resulting image is degraded.